Internal Revenue Service Commissioner Roscoe L. Egger Jr. will take the hot seat on Capitol Hill again this week, when he testifies before the House Ways and Means subcommittee on oversight Thursday about Reagan administration plans to withhold from tax refunds money that delinquent taxpayers owe on loans from the government.

A variety of loans from the Small Business Administration, Education Department, Agriculture Department, Housing and Urban Development Department and Veterans Administration are to be included, and representatives of those agencies also will testify.

Egger will be in a difficult position because, in internal debates within the administration, the IRS has been reluctant to become the debt collector for the federal government. IRS officials are known to feel that withholding taxes that otherwise would be refunded could damage the agency's reputation further, but the Office of Management and Budget has gone ahead and initiated the program.

Subcommittee Chairman J.J. (Jake) Pickle (D-Tex.) wants to know, among other things, whether this action could cause more taxpayers to "drop out" of the system, hiding income to avoid the debt collections. He also wants to know what options taxpayers will have if their refunds are erroneously withheld.

Despite congressional concern about the program, provisions authorizing the IRS to collect the overdue debts was passed by Congress as part of the Deficit Reduction Act of 1984.

TURNING UP THE TIPS . . . A recent U.S. Tax Court case gives an indication of the IRS's dedication to the job of catching tax evaders, even if it involves hard work and hazardous duty. According to court documents, Jill Smith, coordinator of an IRS project to turn up unreported tip income, did yeoman duty in Las Vegas casinos, deriving a complex formula to determine whether waitresses at the Union Plaza Casino were reporting all the tips they were receiving.

The formula was based on "personal observations . . . of tipping practices by customers, conversations with Union Plaza employes, and a general knowledge of the industry," the court decision said. Smith determined, for example, that the average tip was 50 cents (the drinks were free at the gaming tables) and that the average "stiff rate" -- the proportion of customers who didn't tip -- was 50 percent.

Five employes, four waitresses and a blackjack dealer, were accused of underreporting their tips for tax purposes. Under the formula -- which also included adjustments for time of day, lunch breaks and tip-sharing with bartenders -- the five had received a total of $43,308 in tip income rather than the $7,886 they said they had received, and thus owed more taxes than they had paid, the IRS said.

The taxpayers did not disagree with the formula, but contended that it didn't apply to their cases. The Tax Court acknowledged that some of the employes had "special situations" -- several worked principally with gamblers from bus tours, who tip less -- but the court decided that they had received a total of $30,244 in tips.

THE COMPUTER AGE . . . The IRS last week proposed requiring companies with 500 or more employes to submit information about the taxes withheld from workers' salaries on computer tapes rather than on paper. The intent of the new rules is to improve compliance with the tax laws by enabling the IRS to better "match" information from the companies with the employes' tax returns.

The change would take effect in 1987. In 1988, employers with more than 250 workers also would be covered by the regulation, and the IRS has urged companies to comply with the change before then. The proposed change, expected to become final in about six months, also covers reporting of interest income, gambling income, retirement income and tip income, in cases where it is withheld from employes' checks.

RAY OF SUNSHINE . . . Amidst all the difficulties the IRS has had in processing tax returns and mailing refunds this year, the service said this week that a small piece of its computer operations is working well: Because of "increased processing efficiency," the price the IRS charges taxpayers to provide a copy of their tax returns will fall, from $5 to $4.25. The change, effective Oct. 1, also covers requests for specific information from a taxpayers' return, which will cost $2.25 instead of $2.50